The present invention relates to an accumulator fuel-injection apparatus.
An accumulator fuel-injection apparatus is known which stably supplies respective cylinders of a diesel engine with high-pressure fuel stored in an accumulator to improve engine performance in a broad operating region. Even with a fuel-injection apparatus of this type, an abrupt explosion combustion takes place in the initial stage of combustion, causing noisy engine operation and increased NOx in exhaust gas, if the fuel injection rate immediately after the start of fuel injection is too high or if the quantity of fuel injected in an ignition delay period is too large.
As a countermeasure against such a problem, auxiliary injection can be performed prior to main injection in each fuel injection cycle. This shortens the ignition delay period, realizing a reduction in an amount of fuel injected in the ignition delay period by the main injection, whereby the abrupt combustion is prevented to reduce the engine noise and the NOx emission. In case that the fuel-injection apparatus is configured to perform high-pressure injection from the beginning of the main injection following the auxiliary injection, however a satisfactory reduction in noise and NOx emission cannot be always achieved. To attain the ignition-delay reducing effect, a minimum required quantity of fuel should be injected in the auxiliary injection. In order to reduce the fuel injection quantity to the required minimum in the fuel-injection apparatus of a type performing the auxiliary injection at a relatively high injection pressure, the auxiliary injection period must be shortened. This requires high control accuracy. If the required control accuracy is not attained, the fuel injection quantity in the auxiliary injection becomes too small or too large, so that the intended effects of the auxiliary injection may not be achieved, resulting in degraded exhaust gas and fuel economy.
As another means of reducing engine noise and NOx emission, an accumulator fuel-injection apparatus has been proposed that injects fuel at a lower fuel injection rate in the initial stage of fuel injection in each fuel injection cycle. Byway of example, the proposed apparatus has a low-pressure accumulator for storing low-pressure fuel, a high-pressure accumulator for storing high-pressure fuel, a selector valve for selectively communicating the low- or high-pressure accumulator with an injector (fuel injection nozzle) to switch injection rate, and an on-off valve for permitting/preventing communication between a control chamber of the injector and a fuel tank to control injection start/end timings.
An accumulator fuel-injection apparatus of this kind, such as for example an apparatus disclosed in International Publication No. WO98/09068, is designed to control opening/closing timings of an on-off valve for injection timing control and of a selector valve for injection rate switching, so as to carry out only main injection or both main injection and auxiliary injection in each fuel-injection cycle. In addition, there is disclosed a technique of performing low-pressure injection in the initial stage of the main injection followed by high-pressure injection.
In connection with the present invention, the apparatus disclosed in the above publication can perform low-pressure auxiliary injection for a short time, and start main injection when a predetermined period of time has elapsed after completion of the auxiliary injection. Low-pressure injection is carried out in the initial stage of the main injection, and then high-pressure injection is carried out over the remaining period.
Specifically, by closing the on-off valve for injection timing control and the selector valve for injection rate switching, a fuel passage connecting the selector valve and a fuel chamber of the injector is filled with low-pressure fuel and a control chamber of the injector communicated with the fuel passage is supplied with the low-pressure fuel, whereby the injector is maintained in a valve-closing state. When the auxiliary-injection start timing is reached, the on-off valve is opened to discharge the fuel in the control chamber to the fuel tank, whereby the injector is opened to make low-pressure auxiliary injection, and the on-off valve is closed when the auxiliary injection period has elapsed. When a predetermined period of time has elapsed after completion of the auxiliary injection so that the main-injection start timing is reached, the on-off valve is opened again to start low-pressure main injection, and the selector valve is opened in the course of main injection so that high-pressure fuel supplied from the high-pressure accumulator is injected through the nozzle, thereby effecting high-pressure injection.
By conducting the low-pressure auxiliary injection and the main injection including low- and high-pressure injections in each fuel injection cycle as described above, the fuel economy and exhaust-gas characteristic of the engine can be improved. However, there is a demand for further improvement in fuel economy and exhaust-gas characteristic.
The object of the present invention is to provide an accumulator fuel-injection apparatus capable of establishing a proper injection pressure in the initial stage of main injection following low-pressure auxiliary injection, to thereby further improve fuel economy and exhaust-gas characteristic.
In order to attain the above object, an accumulator fuel-injection apparatus according to the present invention comprises a first accumulator for storing high-pressure fuel; a control valve for controlling discharge of the high-pressure fuel stored in the first accumulator toward a downstream-side of a fuel passage; a second accumulator, communicated with the fuel passage on the downstream of the control valve, for storing low-pressure fuel; and fuel control means for opening the control valve for a short time between auxiliary short injection and main injection, for opening the control valve in the course of the main injection, and then closing the control valve in conformity with completion of the main injection, while the auxiliary short injection and the main injection are carried out in this order with an interval through a fuel injection nozzle communicated with the fuel passage.
Desirably, the fuel control means establishes an intermediate pressure in the fuel passage on the downstream-side of the control valve in the initial stage of the main injection, the intermediate pressure being higher than the pressure of the low-pressure fuel in the second accumulator and lower than the pressure of the high-pressure fuel in the first accumulator.
With the accumulator fuel-injection apparatus of the present invention, when the control valve is opened for a short time after completion of the low-pressure auxiliary injection, the fuel passage is supplied with high-pressure fuel from the first accumulator for a short time, so that the fuel pressure in the fuel passage becomes higher than an auxiliary-injection pressure. In the initial stage of the main injection effected through the fuel injection nozzle, therefore, fuel is injected at a pressure higher than the auxiliary-injection pressure, desirably at an intermediate pressure higher than the auxiliary-injection pressure and lower than the pressure of the high-pressure fuel. When the control valve is opened in the course of the main injection, the high-pressure fuel is supplied through the fuel passage to the fuel injection nozzle, so that high-pressure injection is performed.
By injecting fuel in the initial stage of the main injection following the low-pressure auxiliary injection, at a pressure higher than the auxiliary-injection pressure, desirably at an intermediate pressure higher than the auxiliary-injection pressure and lower than the pressure of the high-pressure fuel, the fuel injection quantity in the initial stage of the main injection increases as compared with the case where low-pressure injection is performed in the initial stage of the main injection. Consequently, the fuel quantity to be injected in the remaining period of the main injection decreases by the quantity corresponding to the increase in the injection quantity in the initial stage, and hence the period of the entire main injection is shortened. By injecting the appropriate quantity of fuel in the initial stage of the main injection to shorten the main injection period in this way, the fuel injection ends early and therefore the fuel economy is improved. In addition, unlike the case where high-pressure injection is started from the initial stage of the main injection, excessive fuel supply before ignition is prevented, resulting in reduction in engine noise and NOx emission.
Furthermore, by conducting the auxiliary injection at a low pressure, the required accuracy of auxiliary-injection time control is alleviated as compared to the case where the auxiliary injection is performed at a high pressure, so that the fuel injection quantity in the auxiliary injection may be more accurately controlled to the required minimum, contributing to improvement in fuel economy.
The fuel control means may be so configured, for instance, as to control opening/closing timings of valves for fuel injection timing control and for injection rate switching. This eliminates the need of utilizing a particularly complicated arrangement.
FIGS. 1 to 3 schematically show the states of fuel spray formed by the auxiliary injection, the initial stage of the main injection, and the subsequent main injection, respectively, based on results of a combustion observation experiment the present inventors conducted. In the experiment, observations were made from above a cylinder on fuel spray formed by the fuel injected from a fuel injection nozzle with four nozzle holes. In the illustrations, a small circle represents the fuel injection nozzle, and a large half circle represents half of the cylinder.
In the present invention, low-pressure auxiliary injection is performed prior to main injection, as described above. The auxiliary injection is made in a condition that a piston is on the lower side in the cylinder so that the density of gas in the cylinder is low. This allows fuel spray to easily diffuse outward in the radial direction of the cylinder, however, the diffusion of the fuel spray is restrained appropriately since the auxiliary-injection pressure is low. As shown in FIG. 1, the fuel spray is distributed in an area from the vicinity of the fuel injection nozzle to a radially intermediate part of the cylinder.
As described above, some conventional accumulator fuel-injection apparatuses are designed to inject fuel in the initial stage of main injection at a low pressure equivalent to auxiliary-injection pressure. With this arrangement, since the area to which fuel spray can reach (i.e., fuel spray distribution) overlaps the fuel spray distribution caused by the auxiliary injection, too much fuel may exist in that area. This causes a fear that the area is not supplied with sufficient air required for simultaneous or successive combustion of the fuel sprays formed by the auxiliary injection and the low-pressure main injection. In particular, the air for combustion of the fuel spray associated with the low-pressure main injection is consumed during the combustion of the fuel spray produced by the auxiliary injection, causing shortage of air, if the fuel spray produced by the auxiliary injection is caused to ignite, just before or after the start of the main injection, with the,increase in pressure and temperature in the cylinder due to upward piston movement. At any rate, when fuel is injected at a low pressure in the initial stage of the main injection, the fuel spray and flames caused by the auxiliary injection hinder the diffusion of the fuel spray formed by the low-pressure main injection and hinder the supply of fresh air required for combustion of the fuel spray, preventing a proper combustion of the fuel spray. Thus, black smoke is liable to be discharged from the engine.
In contrast, according to the present invention, the main injection is started in a condition that a fuel pressure higher than the auxiliary-injection pressure is established in the fuel passage on the downstream-side of the control valve, as described above. Since the fuel injection pressure in the initial stage of the main injection is higher than the auxiliary-injection pressure, the fuel spray reaches, as shown in FIG. 2, an area outwardly of the fuel spray and flames formed by the auxiliary injection as viewed in the radial direction of the cylinder. Sufficient air remaining in the outer area permits a proper combustion of the fuel spray. While the fuel injected at an intermediate pressure is scattered to penetrate through the fuel spray and flames formed by the auxiliary injection as described above, surrounding air is involved in the fuel spray, so that the volume of the entire fuel spray increases. This means that the fuel spray diffuses satisfactorily in the cylinder to provide an appropriate fuel spray distribution in the cylinder.
Under such a condition, the fuel injection pressure is changed from intermediate pressure to high pressure. The high-pressure fuel spray properly diffuses in the cylinder as indicated in black in FIG. 3, while increasing, its volume by sucking surrounding air and combustion residues such as soot produced by the auxiliary injection and the intermediate-pressure main injection. Since the diffusing high-pressure fuel spray activates the combustion in the cylinder, the entire fuel spray burns satisfactorily, preventing the production of black smoke. Referring to FIG. 3, the distribution area of the high-pressure fuel spray is dislocated from that of the intermediate pressure fuel spay in the circumferential direction of the cylinder. Presumably, this is caused by a swirl formed in the cylinder.
As described above, according to the present invention, the intermediate-pressure main injection and the high-pressure main injection are performed successively in a condition that the diffusion of fuel spray formed in the cylinder by low-pressure auxiliary injection is restrained appropriately, so as to realize a proper combustion of the fuel spray. This makes it possible to improve the fuel economy and-exhaust-gas characteristic of the engine and reduce the engine noise.
In the present invention, desirably, the intermediate pressure formed in the fuel passage on the downstream-side of the control valve in the initial stage of the main injection by the fuel control means has a value closer to the pressure of the low-pressure fuel in the second accumulator in a lower-speed, lower-load engine operating region, and closer to the pressure of the high-pressure fuel in the first accumulator in a higher-speed, higher-load engine operating region.
Generally, the distance for which the injected fuel flies is longer in a lower-speed, lower-load operating region where the in-cylinder pressure is low, whereas the injected fuel flies over a smaller distance in a higher-speed, higher-load operating region where the in-cylinder pressure is high. In the preferred arrangement, the injection pressure of the intermediate-pressure main injection is varied depending on the engine operating state. Thus, the pressure of injected fuel, the distance for which the fuel flies, and the diffusion state of fuel spray in the initial stage of the main injection are suited to the engine operating state.
Desirably, the fuel control means controls the pressure of the high-pressure fuel in the first accumulator and the pressure of the low-pressure fuel in the second accumulator, based on the operating state of the engine, for example, the speed and load of the engine. In this case, the intermediate pressure formed in the fuel passage on the downstream-side of the control valve in the initial stage of the main injection is suited to the engine operating state.
Desirably, the fuel control means variably controls the period of time for which the control valve is opened between the auxiliary injection and the main injection. The control-valve opening time period determines the intermediate pressure formed in the fuel passage on the downstream-side of the control valve in the initial stage of the main injection. Thus, the intermediate pressure can be varied accurately and easily by the variable control of the control-valve opening period.